Resonator filter structure having equal resonance frequencies

Abstract

The invention relates to a resonator filter structure ( 10 ) for radio frequency (RF) filters, especially a bulk acoustic wave (BAW) filter structure. According to the invention, a resonator filter structure ( 10 ) is constructed with a BAW lattice filter section ( 20 ), in which all of the BAW resonator elements ( 20 - 1, 20 - 2, 20 - 3, 20 - 4 ) within the BAW lattice filter section ( 20 ) have substantially equal resonance frequencies. According to the invention, there are parallel capacitances ( 30 - 1, 30 - 2 ) connected in parallel to the BAW resonators ( 20 - 2, 20 - 3 ) of one branch type of the BAW lattice filter section ( 20 ). Thus, anti-resonance frequency of the respective BAW resonator ( 20 - 2, 20 - 3 ) is tuned. That results in a very narrow passband which corresponds approximately to the difference in anti-resonance frequencies between diagonal and horizontal branches of the lattice filter section ( 20 ). The parallel capacitances ( 30 - 1, 30 - 2 ) are used to tune the bandwidth: the smaller the capacitance, the smaller the bandwidth. Moreover, due to the lattice structure at one port of the resonator filter signal guidance will be balanced while at the other port signal guidance can be unbalanced or balanced according to the application needs.

1. A resonator filter structure on a substrate to provide a passband defined by a center frequency f C , a lower cut off frequency f L , and an upper cut off frequency f U , the resonator filter structure comprising:
 an input port; 
 an output port; and 
 a lattice type filter section between the input port and the output port, the lattice type filter section comprising:
 first resonator elements arranged in series branches as series arms having a resonance frequency f X1R  and an anti-resonance frequency f X1A ; 
 second resonator elements arranged in lattice branches as lattice arms having a resonance frequency f X2R  and a anti-resonance frequency f X2A , wherein all of said resonator elements within said lattice type filter section have substantially equal resonance frequencies and substantially equal anti-resonance frequencies; and 
 means for tuning at least one of said anti-resonance frequencies or one of said resonance frequencies of the resonator elements, wherein the means for tuning at least one of said anti-resonance frequencies comprises a capacitance C connected to each of said resonator elements. 
 
 
     
     
       2. The resonator filter structure according to  claim 1 , wherein said capacitance C connected to each of said resonator elements comprises a parallel capacitance C connected in parallel to each of said resonator elements. 
     
     
       3. The resonator filter structure according to  claim 2 , wherein all branches of said lattice type filter section substantially equal total capacitance at least outside said passband. 
     
     
       4. The resonator filter structure according to  claim 3 , wherein said parallel capacitance C corresponds with
     C =(1 −x )· A·C   AREA , 
 wherein A is an area on said substrate of a bulk acoustic wave (BAW) resonator element in one branch types of said lattice filter section wherein said area has a capacitance per area C AREA  and x is a fraction of said area A, wherein x·A is an area of another BAW resonator element of another branch type of said lattice filter section. 
 
     
     
       5. The resonator filter structure according to  claim 1 , wherein said capacitance C connected to each of said resonator elements comprises a series capacitance C connected in series to each of said resonator elements. 
     
     
       6. The resonator filter structure according to  claim 1 , wherein said resonator elements comprise acoustic wave resonator elements in the form of bulk acoustic wave (BAW) resonator elements or surface acoustic wave (SAW) resonator elements. 
     
     
       7. The resonator filter structure according to  claim 6 , wherein said BAW resonator elements comprise at least a piezoelectric layer having an equal thickness in all of said BAW resonator elements. 
     
     
       8. The resonator filter structure according to  claim 7 , wherein said piezoelectric layer comprises a layer of piezoelectric material of aluminum nitride (AlN) and/or zinc oxide (ZnO) and at least an optional additional dielectric layer of silicon oxide (SiO 2 ). 
     
     
       9. The resonator filter structure according to  claim 1 , further comprising means for impedance matching connected at least to one port of said input port and said output port. 
     
     
       10. The resonator filter structure according to  claim 9 , wherein said means for impedance matching comprise discrete inductive elements and/or discrete capacitive elements connected in series and/or in parallel to said at least one port. 
     
     
       11. The resonator filter structure according to  claim 1 , wherein a first port signal guidance of at least one port of said input port and said output port is balanced. 
     
     
       12. The resonator filter structure according to  claim 11 , wherein a second port signal guidance of at least one port of said input port and said output port is unbalanced. 
     
     
       13. The resonator filter structure according to  claim 1 , wherein the resonator filter structure comprises a radio frequency (RE) resonator structure.